There is no commercially available screen having a resolution able to display both tests necessitating accuracy and tests that display large patterns. There are nevertheless various possibilities for displaying different ophthalmological tests. Certain apparatus uses a screenprinted strip on which all the tests are printed, for example. A portion of the strip is displayed in an active window and a mechanical system moves the strip to display the required test in front of the active window.
The above system is a mechanical system for generating virtually all the screening tests and is designed to enable the screenprinted strip to be changed to adapt it to other countries or uses.
Other, similar apparatus uses tests printed on a drum or turntable, rotation of the drum or turntable presenting the test that it is wished to display in an active window.
Other apparatus uses graphics screens, for example of the cathode ray tube (CRT) type or the liquid crystal display (LCD) type. These screens provide very many tests on a single surface without necessitating any mechanical movement. There are therefore many screening software packages for displaying tests on conventional computer screens. The tests are also very easy to modify or to adapt according to the country, as they necessitate only a software modification. The drawback of these systems is that the resolution of the screens is insufficient to obtain both an image of reasonable size and pixels small enough to produce small optotypes with the required accuracy, for example the accuracy required by the ISO standard cited above.
Moreover, other apparatus is limited to displaying predefined patterns on the screen but can display small optotypes very accurately.